At the present time optical projector systems are widely used in business, educational and consumer applications. For example, slide and motion picture projectors are used to show images from film on screens; projection TV may use one or three LCD panels (Liquid Crystal Display), (LCLV-Liquid Crystal Light Valve) and other projectors may use a computer-controlled LCD. The light for projector systems is provided by a "light engine" which generally consists of a light source, for example, a light bulb, a reflector and one or more lenses to direct the light on the "image gate" such as an LCD panel or film gate. In general it is desirable that the light engine have the following characteristics: (1) the light it provides should be bright; (2) the light engine should not produce heat in excess of its ability to be cooled, for example, by a fan, in order to conserve the life of its bulb and other components; (3) it should produce white light without color spots, which is especially a problem in metal halide arc lamps; (3) the light should be spread evenly over the image gate. Often, in commercial light engines, the light in the center may be at 100% but the light in the four corners may be less than 50%; and (4) the light engine should be physically as small and as low in cost as feasible.
This is an old and crowded field of technology. Hundreds, and possibly thousands, of patents and articles have been written and many types of light engines have been commercially produced. Nevertheless, there still exists the need for an improved light engine for projector systems. Such an improved light engine would spread the light evenly so that the light in the corners of a rectangular image gate ("corner illuminance") is at least 70% of the light at the gate's center; the colors from the lamp are homogenized to produce uniform white light without color spots; and there is reduced wasted light due to "spillage" (light beams which fall outside of the image gate).
In the article "An Uniform Rectangular Illuminating Optical System For Liquid Crystal Light Valve Projectors", Chang et al, Eurodisplay, '96, pgs. 256-260, light from a short arc lamp is gathered by an elliptical reflector and transmitted through a solid glass RPGR (Rectangular Pillar-Like Glass Rod) and then through two lenses to a LCLV (Liquid Crystal Light Valve).
In addition, a number of other articles and patents have described solid glass, plastic or dielectric filled light pipes for projection systems including U.S. Pat. Nos. 5,146,248 (column 6, lines 35-51 ); 4,813,765 (column 1, lines 40-42); 5,696,865 (column 4, lines 61-65); and 5,634,704 (column 8, lines 55-58).
A solid light pipe, when used in a projection system to transmit light from a light source to an image gate, presents a number of problems. If the light source produces a large amount of heat, such as an arc lamp, the light pipe may have to be made of Pyrex (TM of Corning) or other high temperature resistant (low thermal expansion) glass. However, the light transmission of such high temperature glass is poor compared to regular optical glass, and consequently light would be lost and the efficiency of transmission of light would be unsatisfactory. In addition, although optical glass or optical plastic appears clear and unblemished to the naked eye, it generally has microscopic sized bubbles and lines. Those microscopic sized imperfections cause an unpredictable and undesirable color shift in the transmitted light.
It may be difficult, or impossible, to meet a typical specification for color uniformity of .+-.200.degree. K. due to color shifts caused by the non-homogeneous solid transparent material of the light pipe. Furthermore, a solid light pipe, especially if made from glass, adds to the weight and expense of the light engine. It requires special mounting, for example, by screws having pointed ends, and the mounting may be delicate and expensive.
U.S. Pat. No. 3,774,995 (Class 359-869) discloses a reflector having sections of the surface with different curvatures, including an ellipse.